Digital x-ray system

ABSTRACT

A radiology system for storing, processing, and displaying two-dimensional image data derived from an x-ray source. A visible x-ray image or sequence of images is scanned to produce a signal representing the intensity of the images, and this signal is encoded to provide a data compressed digital signal which is stored and processed to yield displayed images of enhanced quality in which image sequence and processing intervals may be in real-time. The encoded signal is of a format to reduce the amount of data stored in memory, and temporal and spatial averaging and variable length coding are employed for efficient image data processing. Apparatus is provided for defining a mask image, from which other image data is subtracted to yield an enhanced image. The system can include means for providing a control signal representing the level of radiation absorption by a body, and adjusting the intensity of x-ray radiation in accordance with the absorption level.

FIELD OF THE INVENTION

This invention relates to radiology systems and more particularly to asystem for storing, processing, and displaying image data derived froman x-ray image.

BACKGROUND OF THE INVENTION

Radiology systems are known in which a visible image is provided inresponse to the selective absorption of x-rays transmitted through abody from an x-ray source. In order to enhance the visible image or toderive information therefrom which is not readily perceived by viewingof the image, systems have been developed for storing data derived fromone or more visible images and for processing the stored data to providean enhanced display of the processed image data. In general, a visibleimage is provided on the screen of an image intensifier in response toreceived x-ray radiation, and a video camera is employed to scan thevisible image and to produce a video signal which is processed toprovide a display of the x-ray image, or stored for subsequentprocessing and display. Usually, a sequence of images is stored andprocessed to provide enhanced image quality and sufficient image datafor comparative analysis.

The ideal system would provide a high quality displayed image forviewing and analysis in real time. Present systems do not achieve suchideal performance, since known systems which provide an image in realtime or substantially real time suffer relatively poor image quality,while systems having good image quality require significant processingtime in order to generate the displayed image.

SUMMARY OF THE INVENTION

The present invention provides a radiology system for storing,processing, and displaying two-dimensional image data derived from anx-ray image. The x-ray image is preferably in the form of a visibleimage or sequence of images provided by an image intensifier screen. Avideo camera is operative to scan the visible image and to provide anelectrical signal representative of the intensity of each pixel area ofeach image, this electrical signal being digitized, encoded, anddigitally stored. The stored data is processed and decoded to generatedisplayed visible images of the processed image data. The x-ray sourcecan provide x-ray radiation of controllable intensity in response tochanges in the electrical signal derived from the x-ray image. Theelectrical signal can also be processed to provide a measure of theaverage intensity of the x-ray image. The encoded signal is of a formatto reduce the amount of data stored in memory, and temporal and spatialaveraging and variable length coding are employed for efficient imagedata processing.

DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of the system;

FIG. 2 is a crosspoint matrix illustrating the cross bar of FIG. 1;

FIG. 3 is a time plot showing the absorption due to the bolus;

FIG. 4 is a graph showing the required additional number of digitizationbits for three mathematical functions;

FIG. 5 is a graph showing the entropy associated with three analog todigital converter mathematical functions;

FIG. 6 is a graphical representation of the digitized signal in squareroot relationship to the input signal;

FIG. 7 is a particular embodiment of the square root converter of FIG.6;

FIG. 8 is a graph showing the region of truncation for one embodiment ofimage signal compression;

FIG. 9 is a block diagram of a Huffman encoder;

FIG. 10 is a block diagram of a Huffman decoder;

FIG. 11 is a block diagram of first phase system operation;

FIG. 12 is a block diagram of phase II of the system operation;

FIG. 13 is a block diagram of phase III system operation;

FIG. 14 is a block diagram of phase IV of the system operation;

FIG. 15 is a block diagram of phase V of the system operation;

FIG. 16 is a statistical plot of the data showing truncation pointsthereupon;

FIG. 17A shows an alternate configuration of phase III;

FIG. 17B shows a second alternate configuration of phase III; and

FIG. 17C shows a third alternative embodiment of phase III.

DETAILED DESCRIPTION OF THE INVENTION General System Configuration

The general block diagram of the digital x-ray system 100 is shown inFIG. 1. The system comprises a controlled x-ray source 102 producing anx-ray beam (either continuous or pulsed) whose intensity is controlled.For standard x-ray tubes, the beam intensity can be controlled byadjusting the beam current by a signal from system controller 104. Asystem console 105 allows the system operator to the sequence ofoperation as desired, as well as specified operator or physicianinteraction, as discussed later. The controlled x-ray source 102projects the x-ray beam 108 through a subject to be examined 110 and isreceived by an image converter and intensifier 112 which produces avisble image thereupon. The gain of the image converter and intensifiertube is adjusted to provide an improved signal-to-noise ratio forparticular level of x-ray radiation received, the gain adjustment beinginversely related to the x-ray radiation intensity. The visible image isconverted to an analog video signal V by a video camera 114. The videosignal is received by a low-pass analog filter 118 to reduce noise andeliminate aliasing. The filter 118 output is received by a combinationanalog/digital converter (ADC) 122 and an encoding look-up table 121.The look-up table 121 provides linearity correction for high brightnesssignals received by the video camera 114, convert the digitized signalto another format and provide callibration adjustment in concert with aknown calibration signal generated by the system controller 104 andreceived by the analog/digital converter 122. In a generalized form, theconverter 122 may have any format (or a format x) signal as an output,which is subsequentially converted to a squre root value of the videosignal V, linearized by encoding look-up table 121. The digital outputsignal from look-up table 121 includes a sufficient number of levels ofdigitization to sufficiently represent all necessary picture informationwithout degradation, distortion or addition of unwanted noise, asfurther described below. The filter output signal is in turn received bya processor 124. The processor 124 includes an arithmetic unit 126having a plurality of internal registers, and encoding and decoding datalook-up tables and adders/subtractors and is connected to a memory 136,137, 138 and 139 (memory 139 optional). All memories 136, 137, 138 and139 are interchangeable and will alternately bear labels "buffer," "mask" and "image average", etc. according to the data stored therein.For instance, if a x-ray pulse is generated, by x-ray source 102, andphased with some external event (such as a cardiac gating signal), and aprogressive scan TV camera is used as the video camera 114, memory isused as a temporary buffer memory to provide as an output to both fieldsof the TV signal in phase with the operation of the system, whichprocesses images in interlaced mode, at 30 frames per second. Also, datatransferred to or from the disc 142 will be temporarily stored in thebuffer memory. The processor 124 is controlled by a system controller104 and cross-bar 150 through switch register 152 wherein eachinterconnection path between the elements of processor 124 as well asthe time arithmetic unit 126 is capable of being restructured on aframe-by-frame basis, typically at a rate of thirty times a second,depending on the particular process performed. These processes arediscussed in relation to system phases, discussed below. The digitizedsignals are received by the processor 124 through cross-bar 150including a signal input port 146. The processor 124, through itsassociated look-up tables, as well as the memories 138, 137 and 136 andthe disc 142 through the data compressor 140 are interconnected bycross-point 150 to the signal input port 146 which form data paths. Thecross-point 150, shown in FIG. 2, is a matrix of bi-directional datapaths 153, 154 whose intersections include data switches 155 controlledby switch register 152 and system controller 104 shown in FIG. 1. Allprocessing steps (shown by example later as steps in system Phases I-Voccur at integer number of image frames, which occur 30 times persecond, or once every 33 ms. Therefore, as data is transferred among thevarious system elements (including the signal input port 146, thearithmetic unit 126, the memories 136, 137 and 138, the disc datacompressor 142 and the video display 164 through controller 163) theactuation of the respective switch for the respective processing stepoccurs at the frame rate, or 30 times per second. Each of the switchedsystem elements is connected both to data paths 153 and 154, whichpermits additional paths to be provided when no connection conflicts (aconflict such as two elements communicating to one memory) exist.Additionally, the switch elements 155 are reed relay or appropriatesolid state data switch elements known in the art. The processor 124 andits above-mentioned elements are discussed in greater detail below inrelation to various interconnections of the system elements. Connectionof the elements through the cross-point 150 will be mentionedspecifically hereafter. The selection of the exact switch 155 such as155A, 155B or 155C is determined according to the desired path (shown inFIG. 2) between the respective elements. Several x-ray images are storedon a magnetic disc when the data is encoded, decoded and compressed bythe data compressor 140, connected between the cross-point 150 and thedisc 142. The signals stored on the disc 142 may be retrieved andrestored for subsequent signal processing. The results of the systemoperation or particular intermediate steps are displayed on a videodisplay unit 164 having a picture element (pixel) matrix of 512 by 512,or 1024×1024.

X-ray Intensity Control

In the operation of the above-described digital x-ray system, and inparticular as applied to angiography, it is necessary to increase thecontrast of the blood vessels over the background of the rest of thebody tissue. Typically, the blood vessels are contrasted byintravenously injecting a radiation opaque solution, such as an iodinedye, into the bloodstream. The dye, while still a non-fully dilutedmass, is called a bolus. When the bolus enters the area of interest,such as heart blood vessels, before the dye becomes fully diluted, theheart blood vessels are contrasted from the surrounding tissue. Thex-ray image produced before the bolus arrives is proportional to thenumber of x-rays N, times the attenuation by the tissue e⁻μ, where μrelates the absorption of x-rays by the body tissue over the distancethe x-rays travel in the body tissue and, in general, is used as areference, or mask image M. The image I_(J), during the bolus, is aproduct of the number of x-rays N and the absorption of x-rays by thedye in the heat blood vessels e⁻β and the tissue absorption e-.sup.μwritten together as e.sup.[-μ-β]. The preliminary mask image M is formedas an average during the time shown in FIG. 3 between the time periodsmarked 202 and 204, or at a particular time 203, or at a weightedaverage of times 205 and 207 as discussed in Phase IV of FIG. 14. Since(1) the images are a result of the x-ray absorption by the tissue andthe dye, and (2) the absorption is an exponential component, an image ofhigher contrast showing more useful information formed by subtraction ofthe two components of absorption (μ and μβ) as I_(J) -M or Δ_(I). Thebolus increases in presence to a maximum intensity, thereafterdecreasing at time period 206 in level to where it is circulated,absorbed and diffused to the remaining part of the body.

The definition of the difference (I_(J) -M) images Δ_(I) derived isimproved according to the present invention by modulating the intensityof the x-ray source 102 according to a determination of the onset of thebolus in the cardiac area, typically as described by a transition pointat 204 of FIG. 3. A fundamental constraint of the x-ray system operationis the maximum limit for radiation to be applied to the subject (over agiven period of time) without causing skin damage and/or maximum powerdissipation of the x-ray source cathode. It is observed that until thebolus arrives at the viewing area at time 204, the images producedearlier have a limited amount of useful information concerning bloodvessels. Since the most useful information is derived during the periodof the bolus (202 to 204) the period before the arrival of the bolus(202 to 204) can be adequately observed with reduced x-ray intensitywithout an unacceptable decrease in image quality. The analog signal Vderived from the low x-ray intensity image sequence during interval202-204 of FIG. 3 is normalized to yield comparable intensity with otherimages of different relative x-ray levels.

Therefore, according to the present invention, assuming a maximumaverage x-ray intensity from the x-ray source 102 over the period 202 to206 of FIG. 2, a reduction of intensity over the period 202 to 204permits an increase in x-ray intensity during period 204 to 206.Accordingly, a below average intensity of the x-ray source 102 isprovided by system controller 104 before the onset of the bolus (202 to204) and an above average intensity of the x-ray source is providedduring the bolus (202 to 204) thereby increasing the difference imagebut without increasing the risks of overexposure.

During the first interval 202 to 204, the image derived from a reducedx-ray level is observed and measured by the system of the presentinvention whereupon a threshhold image intensity level is calculated,the threshhold typically being the average intensity. At some time,shown typically as 204 (which varies in time according to the physicalstructure of the subject examined, the placement of the injection ofiodine within the bloodstream, and other physical and circulatoryparameters unique to each subject), the intensity of the images duringthat interval will reflect the occurrence of an increase in detectedabsorption (μ); concurrently, the system controller 104 will compare thecurrent image intensity with the calculated level, and provide a signalto the x-ray source 102 to increase the x-ray emissions to a levelsignificantly above normal when the comparison shows the current imageexceeding the calculated level. The increased x-ray intensity continuesuntil the bolus diminishes, shown as the time 206, where the radiationfrom the x-ray source 102 may be diminished or terminated entirely.

As mentioned earlier, a base reference or mask image M is determinedwhich is then subtracted from the image I_(J) derived during theduration of the bolus. The averaging of all of the images producedduring the period preceding the bolus (202 through 204) offers adeceptively attractive possibility of a large number of images; however,during this period which spans a significant time period, there islikely to be a substantial amount of patient or tissue movement,blurring or significantly altering the accuracy of the mask if derivedfrom the average of all images during the time period 202 to 204.Therefore, in accordance to the present invention, one preferred area atwhich to derive the mask image M is at the onset of the bolus at thetime 204. In one embodiment of the present invention, the first imageproduced from an elevated x-ray source emission level at the time 204 isdefined as the mask image M. A mask produced in this manner has animproved image (of background tissue information) as a result of thehigher intensity of the x-ray source 102, while at the same time havinga minimal contribution from the radiation opaque substance in the bloodsystem thereof.

Alternately, the mask image M can be derived from a minimum number ofimages I_(J) (typically 8) which occur just before or just after thebolus, as shown in FIG. 3, 205 and 207, respectively. The optimum maskimage M, to be used in post-processing (described below) is derived froman average, in FIG. 3 at 208, of the images neighboring the bolus,properly averaged with the average value of the bolus itself.

The image produced by the image converter and intensifier 112 isconverted to an analog video signal at lead 116 of FIG. 1 by a videocamera 114 or other image-to-voltage conversion device to provide avoltage relating the particular image intensity of a given area on theimage converter and intensifier screen. The video camera 114 in FIG. 1provides output at 30 frames per second interlaced or 60 fields persecond of the target 230 of the image converter and intensifier 112,each having a resolution of at least 483 visible horizontal lines (in a525 line system) and a visible horizontal resolution of at least 512points. The 1024×1024 pixel display would require a correspondinglygreater resolution video display. Additional horizontal lines and scanintervals in the scan signal which would otherwise, in addition to thevisible image provided by video camera 114 are included to provide thenecessary timing and syncronization signals according to signal formatsand described by standard video standards such as the NTSC Americanstandard for the 512×512 pixel display. The system hereinafter describedincorporates such standards as applicable but it is not necessarilylimited thereto.

Moreover, additional image information, such as patient informationserial numbers, user comments and location coordinates may be includedin gray level scale somewhere on the screen.

The analog filter 118 comprises a low-pass 3-pole filter with a roll-off(-3 dB point) at 1/2 of the digital sampling rate, which corresponds toa roll-off frequency of 5 MHz for 512×512 and 20 MHz for 1024×1024 pixelimage. The filter 118 is incorporated to decrease noise and reducealiasing errors. The particular circuitry for 3-pole low-pass filtersare known in the art.

Digitization

The signal V from the output of the filter 118 is digitized into a wordof bit length Z. The subsequent digital processor 124 (discussed below),display and storage elements place a high premium on the number of bitsper sample of data (in terms of system price). It is therefor highlydesirable to minimize the number of bits Z thereof for each sample ofinformation. The bit length Z is to be minimized without any loss of theinformation contained in the signal V. The interval between oneincremental value of the digitized code and the next, called thedigitization interval δ, must be small enough in comparison to theintrinsic uncertainty of the signal (mainly related to the photonquantum noise) to assure any information lost is below the noise levelσ_(s) of the uncertainty. The level of intrinsic uncertainty isselectable according to the particular system design chosen and isinversely related to the variable η. Therefore, the relationship betweenthe level of RMS noise σ_(s) of the signal V (or its transform S) isrelated according to the formula:

    σ.sub.s =ηδ                                (5)

Furthermore, this relationship must exist over the entire range of thesignal V. That is:

    σ.sub.min,s (V)≧ηδ                  (6)

or equivalently, ##EQU1##

In the present invention, the digitized signal S is proportional to thesquare root of the analog signal V, S=√V, the relationship which, whendifferentiated and substituted in Equation 7, becomes ##EQU2## Assumingthat the noise inherent in the analog signal V is mainly due tostatistic of the photons which create the images from which the analogsignal V is derived according to the equation V=αNe⁻μ, shown earlier,the noise σ_(V) is defined as: ##EQU3## which, when substituted inEquation 7 becomes: ##EQU4## where α is a proportionality constant.

For the transformation S=√V, adopted in the present invention, σ_(s) isindpendent of the value of attenuation e⁻μ, i.e. constant over the rangeof analog signal V. This permits the technique of the present inventionto minimized the number of bits Z (independent of the signal S) indirect relation to a given value of η. By linearly digitizing S(assuming S=√V) between its maximum value (αN) and its minimum value(αNe⁻μ max), the width δ is: ##EQU5## substituting into Equation 10, therelationship becomes ##EQU6## which when solved for Z becomes: The valueof η is selected to limit the additional uncertainty introduced by thedigitization, called the quantitization error Q, to a predeterminedvalue. Assuming that the distribution of the actual value of analogsignal V with any digitization interval is uniform, the quantitizationerror Q (expressed as equivalent RMS noise) is: ##EQU7## therefor, thetotal signal S uncertainty σ_(S),D (expressed in RMS equivalent noise)after digitization is: ##EQU8## whereas before the digitization, σ_(S)=ηδ. The added relative RMS noise including the quantitization error is:##EQU9## which for specific values of η, (δ=1) yields (in percenterror): for η=1/2 ##EQU10## for η=1 ##EQU11## for η=2 ##EQU12##

As mentioned above, the transformation S=√V results in a minimum valueof Z, primarily as a result of the independence of σ_(S)(V) to the rangeof V in Equation 7. For other transformations of S=f(V), such as thoseshown above, σ_(S)(V) assumes different values over the range of μ, andtherefor of the analog signal V. Any linear digitization of the signal Sresults in excessive resolution of the digitized signal, where σ_(S),MIN>σ_(S), for S=f(V) as compared to S=√V in the present invention. Thisexcessive resolution obviously results in a higher value of Z. Alternateexamples of the transformation S=f(V) which may be compared to theEquation 13 for derivation of the value of Z, are shown below.

For S=V: ##EQU13##

For S=log_(G) V: ##EQU14## Since N and η will be selected constantsaccording to the design of the particular x-ray system, it can be easilyshown that Z for these two alternate examples are always higher than theZ required for the transformation S=√V, over any range of the analogsignal V. Moreover, as the dynamic range of the signal V increases,according to a greater difference in the absorption μ, the size of thenumber of bits Z is bounded only for the transformations s=√V, as shownin FIG. 4. The size of Z varies significantly according to the thirdterms of the Equations 13, 18 and 19 which incorporate the value ofμ_(max) into the mathematical relationship. The results of the thirdterm of each respective equation is plotted along the vertical axis 250wherein the value of μ is plotted along the horizontal axis 252. Thethird term contribution of the linear transform is shown by curve 254wherein according to Equation 18, increases from minus infinity (-∞) fora μ of zero, and increases rapidly as μ increases. The contribution ofthe third term of the logarithmic transform shown in Equation 19 isshown by curve 256. This term provides a value which increases fromminus infinity to a constantly increasing value as μ increases from zeroto infinity. By contrast, the present invention incorporates a bitlength whose third term contribution, as shown in the square roottransformation of Equation 13, and demonstrated relative to a range of μby curve 258. According to the present invention, the contribution tothe number of bits Z required is bounded to a value less than one underall signal conditions or extremes of the value of μ.

The analog video signal V from camera 114 of FIG. 1, filtered by filter118 and subsequently encoded and compressed by system elements 121 and122 and transformed into a digital signal S. The analog video signal Vis converted to digital signals of a particular generalized format(format x) by converter 122 and encoded by encoding look-up table 121 sothat the resulting digital signal accurately represents the incomingvideo signal under all operating conditions, with a minimum number ofbits per converted sample. The two elements 121 and 122 may be combinedin a single digitizer 125. In one embodiment of the present invention,the digitizer 125 of the x-ray system 100 shown in FIG. 1 typicallyincludes an analog-to-digital converter 122 and 121 having acharacteristic of producing a resulting digital signal equal to thesquare root of the incoming analog signal.

A graphical representation of the digitized signal S in square rootrelationship to the input voltage signal V is seen in FIG. 6. On thehorizontal axis 260, two voltage signals are shown. The signal at thelower extreme of the voltage distribution 262 shows a much narrower peakrelating the σ of voltage distribution, thus (in the linear scale)requiring a higher number of bits for accurate representation. At thehigher extreme, a voltage distribution 264 exhibits a broader voltagespan for the same number of σ. These signals are related to outputsignals along the vertical axis 266 according to a transfer curve 268having the square root function. Therefore, it is seen that the narrow(linear) voltage distribution 262 is expanded somewhat to a distributionshown as 270, while the broader voltage distribution 264 (in the linearscale) is compressed somewhat and shown as the distribution of 272 sothat the signal distribution 270 and 272, as well as the relativedistributions of intermediate voltage signals, are expected to be ofapproximate constant width relative to the number of bits Z available toencode the respective digital signal V amplitude, while othercompression or encoding techniques, such as the linear encoding, asdiscussed above, will require a greater number of bits Z per word tomaintain an adequate representation of the narrow (and lower amplitudeor higher μ) voltage distribution of 262 of S while maintaining adynamic range necessary for the voltages possible of the voltagedistribution 264. Similarly, an alternate logarithmic relationship(S=log_(G) V) requires a greater number of bits at the upper amplitudevoltage distribution 264 (for lower μ) of analog voltage signal V.

In particular, the square root digitizer of the present invention, underthe conditions where (1) N≃5000, (2) μ_(MAX) ≃6.9, and (3) Z isapproximately 8, whereas a logarithmic digitizer would require Z=10 anda linear digitizer would require 12 bits per analog sample (per word),according to solutions of equations (13), (18) and (19), above.

A particular embodiment of the square root converter implementing theabove-mentioned square root relationship between analog input anddigital output shown now in FIG. 7.

The analog signal V is received by a sample and hold (S/H) 280 toprovide a stabilized sample of V over the conversion period for eachconverted sample, the sample period in the present invention being about100 nanoseconds. The stabilized sample signal from the S/H 280 isreceived by linear analog-to-digital converters 282 and 290, providing afirst digital word (typically=6 bit) and a second digital word,respectively. The first digital word, which provides a generalindication of the overall level of the signal S, is received by aprogrammable read only memory look-up table (PROM) 284 producing anupper digital code and a lower digital code, to be converted to an upperanalog signal and a lower analog signal by digital-to-analog converter(DAC) 286 and 288, respectively. The upper analog signal and the loweranalog signal is recieved by A/DC 290 as external upper and lowerreference signals respectively, which permit the A/DC to convert theanalog signal from S/H 280 to a digital output signal along a piece-wiselinear transformation curve (approximately curve 268 of FIG. 6) having64 linear segments corresponding to the 6 bit value of the first digitalword (2⁶ =64).

Thereafter, the A/DC 290 second digital (output) word and the firstdigital word may be directly combined to form the square rootrepresentation (S) of the analog input (V), or both digital words arereceived by another PROM or random access memory (RAM) look-up table292. The look-up table 292 provides a transform into alternate formats(beside S=√V), linearity correction for high contrast or brightnessvideo signals and calibration adjustment. The calibration is provided bya predetermined analog input V_(t) generated by the system controller104, and received by the digitizer 125 or filter 118 whilesimultaneously reading the converted digital output produced by look-uptable 292, which should directly correspond to the test signal V_(t).Errors, corresponding to differences between the values of V_(t) and thelook-up table 292 output, are compensated by adjustments to the contentsof the look-up table 292 by the system controller 104.

Image Storage

To provide useful angiographic information, the x-ray system 100 mustpreserve a number of images in sequence beginning prior to the onset ofthe bolus of interest, before the period between time 204 as shown inFIG. 3. Accordingly, the present invention can record at least six toeight images per second during the process. The analog video signals V,derived from the visible image, and square root encoded and digitized,are further compressed by the use of a variable length truncated Huffmancode. In addition, before the Huffman encoding, the technique accordingto the present invention, the technique according to the presentinvention transforms a particular word W_(JK) representing the pixel Kof the image J into a new word W_(JK) *

    W.sub.J,K *=W.sub.J,K -W.sub.J,K-1 -(W.sub.MASK,K -W.sub.MASK,K-1) (20)

thereby recording the result of a double incremental difference, thefirst one between adjacent pixels of the same image (spacial compressionof the information) and the second one being between this spaciallycompressed value and the corresponding compressed value of the maskitself (time compression of the information).

The spacial compression can be further improved by using more than oneof the prerecorded pixels of the image itself, such as one of theneighboring pixels.

The mask preferred over any previously recorded frame for use intemporal compression of the information, for the following reasons.First, the system design is simplified. Second, the overall noise of theword W_(JK) * is reduced, as based under the assumption that the levelof the noise of the element (pixel) of the mask has been significantlyreduced by the proper averaging. Third, the image is accuratelydecompressed (exploded from compressed format) without altering thereference mask. Images are gathered and processed in this manner(including all of the above-mentioned methods of data compression) andsequentially stored on the disc 142 until the necessary number of imagesare recorded. The preferred media for digital image storage is amagnetic disc having a data transfer rate of at least 10⁶ informationbytes per second (1 byte≧8 bits)and capacity of up to 60 mega bytes.

A typical representation of the expected difference signal resultingfrom the subtraction operation thus described is shown in FIG. 8. Sinceeach image I_(J) has a possible 2⁸ or 256 different codes, the range ofthe incremental difference signal Δ_(I) extends for all the practicalcases from -2⁸, as shown at 300, to a+2⁸ shown at 302 along thehorizontal axis of the curve. Since most of the information on imageswill be slightly different from that of the mask, and further a verysmall incremental difference for adjacent pixels within the precedingsubtracted image, most of the results of the image differencescalculations will reside around zero (marked 306) having the highprobability 304 shown as extending along the vertical (probability)axis. It is realized according to the present invention that the numbershaving the higher probability (those having the higher value along thevertical or probability axis) be represented in an encoding scheme tohave a fewer number of bits. Conversely, the signals of lesserprobability would be represented by a longer code or bit sequence. Thistechnique is known as variable length coding which includes Huffmancodes known to the art. A possible troublesome consequence of Huffmanencoding exists when an extremely low probability signal occurs, as mayexist towards the signal extremes indicated at 300 and 302, such thatthe resulting Huffman code can be a seemingly very high number of bitsfor a single code value. Thus, for a signal of a finite resolution, asupposedly compressed encoded number may exceed the number of bits ifthe signal were directly recorded according to the present system.Therefore, the Huffman code is truncated at some point in the signalsample value (along the horizontal axis 312) to insure the maximumlength of any H code does not exceed a given number (for instance 12).Two points labeled 308 and 310 relate to the negative and positivetruncation points of the probability distribution 304. The system nowoperates in a manner to encode the values of input signals between thelimits marked at 308 and 310 according to a particular Huffman code, andthose signals which reside outside of the above-described interval,being between 300 and 308, and between 310 and 302, respectively, willbe directly transferred in a bit-for-bit value in the storage discmedia; without special encoding, an additional service word will also berecorded adjacent to the nonencoded signal, thus indicating the signalas being nonencoded.

Another truncation scheme can be implemented, as already known in theliterature, in which the code of the portion 370 of FIG. 16 of the xaxis are represented by a "double" Huffman code, the first being aservice code defining the portion 370 (and belonging to the set of theuntruncated Huffman codes), the second being a proper code belonging tothe set of the untruncated Huffman codes.

A typical hardware implementation of the Huffman encoding scheme isshown in FIG. 9 corresponds to the encoder part of data compressor 140of FIG. 1, included therein. The incoming signal W_(JK) * is received by9 bit latch register 330 according to clock signal D and in turn by theprogramable read only memory (PROM) 332 and 334. The PROM 332 serves aslook-up table generating the most significant bits of the Huffman code,up to a maximum of 8, and the PROM 334 serves a look-up table generatingthe 4 least significant bits, if need, and an additional 4 bit codeproviding the information about the length of the actual H codegenerated. Therefore, the number of shift steps is defined by the lower4 bits of the PROM 334 along output lead 340 and received by clockcontrol circuit 346. The clock control circuit 346 produces a series ofclock pulses, equal in length to the value of the shift intervalproduced on lead 340. This clock B signal is received by shift register335 to produce a serial bit stream, of length equal to the number ofclock B pulses, from the encoded word stored on latch register 342. Theclock control circuit is synchronized to the disc data rate by discclock signal received on lead 348. When a low probability input W_(JK) *data word value is detected, by PROM 332 and PROM 334, the PROM 332outputs the service character (typically 3 bit code) into the latch 342,and, upon control of the clock A into the shift register 335, and, inthe same time, the uncoded word W_(JK) * is transferred into the latch331 and the shift register 333. Upon completion of this transfer, thecontrol clock, as controlled by the special 4 bit length code receivedby the PROM 334, first sends 3 shift step to the shift register 335, andthen 9 shift step to the shift register 333.

A block diagram of a decoder receiving the Huffman encoded signal isshown in FIG. 10 and is located within data compressor 140 of FIG. 1.The encoded serial bit stream is received by shift resgister 350 fromthe disc 142 of FIG. 1 according to disc clock signal on lead 351 at itsdata input. The shift register 350 provides a 12 bit parallel out datastream on lead 354 which is received by both PROM 356 and 360. When thestream of bits coming from the disc is shifted in such a way that thevalid Huffman code is present with its most signficant bits (MSB) at thefirst address of the PROM's 360 and 356, the PROM 356 sends the commandVA to the clock control 352. The clock control thus latches in the latch357 the undecoded 9 bit word W_(JK) *, 8 bit of which are generated bythe PROM 360 and the remaining one by PROM 356. The word W_(JK) * isthen latched into the latch 355 and sent to the cross-point 150 by theoutput clock C of the clock control 352, whose output clock C alsoindicates that valid data are entering the cross-bar bus.

In case the valid Huffman code detected is, in fact, the 3 bit servicecharacter, indicating that the following 9 bits represent the uncodedW_(JK) *, the PROM 356 output both VA and SE to the clock controller352. Upon receiving VA and SE, the clock controller 352, transfer, viathe command clock B, the 9 bits representing the uncoded word W_(JK) *to the latch 359, and then to the latch 355, as above. The control clock352 also provides that during the next 12 shift steps, the clock A isnot activated (whether or not the input is received by the PROM 360 and356), thus preventing ambiguous situations, as may result during these12 shift steps where the undecoded data words may be interpreted as avalid Huffman code.

It is important to note that the spatial and temporal compressiontechniques of the present invention result in a reduction of the entropyof the distribution of the signals of the information to be encoded, andthat this reduction of entropy results in the lower bit rate of theinformation encoded, by making use of the variable length, truncatedHuffman codes.

It is also important to note that the data to be encoded inherentlycontains a certain amount of noise, and that for this application isessentially due to the photon statistic.

Whereas the above-referenced temporal and spatial compression techniquesresult in a reduction of the entropy of the information in reference toa mask image M, such techniques are not able to result in anycompression of the noise associated to the data, due to the structure ofthe noise itself which assumes independent values for each of thepixels.

In addition, the temporal and spatial subtraction technique used toreduce the entropy of the information, will result in an increase of thenoise of the data W_(JK) * to be encoded.

Specifically, for the word W_(JK) *,

    W.sub.J,K *=W.sub.J,K -W.sub.J,K-1 -(W.sub.MASK,K -W.sub.MASK,K-1) (21)

the RMS noise σ_(JK) *, will be: ##EQU15## Assuming that the noise ofthe words representing the pixel of the mask has been reduced to anegligible value by an averaging process, and assuming that the RMSnoise is equal to σ_(N) for all the pixels of the image J, it results:##EQU16## The histogram of the words W_(JK) *, shown in FIG. 8, has avariance σ_(H) ², given by the relation:

    σ.sub.H.sup.2 =σ.sub.I.sup.2 +σ*.sup.2 =σ.sub.I.sup.2 +2σ.sub.N.sup.2                (24)

in which the term σ_(I) represents the variance of the theoreticalhistogram information and the term 2σ_(N) ² represents the additionalvariance due to the noise intrinsically carried by the informationitself.

The entropy of the histogram of the words W_(JK) * can be quantified,under the hypothesis below, to be: ##EQU17## Assumed are the following:first, the histogram of the word W_(JK) * has a gaussian shape. This isverified in practice for most cases; second, both σ_(I) and σ_(N) areexpressed in number of digitization intervals δ; and third, δ is notlarger than √σ_(I) ² +2σ_(N) ². This is automatically verified from thefact the digitization used σ_(N) ≧ηδ with η≧1.

It can also be noted that in most practical cases σ_(I) is comparable to(in general lower than) σ_(N), so that it results that the entropy ofthe histogram of the words W_(JK) *, H_(W).spsb.*, heavily depends uponσ_(N), and, consequently, so does the efficiency of the compressionprocess.

As a significant feature of this invention, it will be shown thatencoding the data W_(J),K in square root format, a minimum value of theentropy H_(W).spsb.* efficiency of the compression process is granted.

In order to do so, a new parameter is defined H_(N), which is theentropy of the noise associated to the words W: ##EQU18## It should benoted that the histogram of the words W_(JoK) *, for an image I_(Jo),representing the same patient situation described by the mask itself,is: ##EQU19## and this relationship provides a physical interpretationof H_(N).

Furthermore, it is possible to write H_(W).spsb.* in a form thatincorporates H_(N) instead of σ_(N). This form is as follows: ##EQU20##and shows how H_(W).spsb.* depends upon H_(N).

For the data W's encoded in square root format, the entropy is:##EQU21##

For the data W encoded in linear format, the entropy is: ##EQU22##

For the data W encoded in logarithmic format, the entropy is: ##EQU23##FIG. 5 shows H_(N) on axis 328 as a function of μ on axis 326 for thethree formats log 326, linear 332 and square root 324. From the figure,it can be seen that the values of H_(N) for the logarithmic format 326and the linear 332 format, are higher in those portions of the range ofthe signal in which these format results in a digitization finer thanneeded, whereas, for the square root format 324, for which the level ofthe digitization is constant and can be kept at the minimum wantedlevel, over the entire range of the signal, H_(N) is constant andminimum, over the range itself.

As a result of the above, the average bit rate per word b/W, encoded inthe variable length, truncated Huffman codes, will heavily varydepending upon the format of the words to be compressed.

Assuming, as an example:

(1) σ_(I) =1

(2) μ (average)=3

(3) μ_(max) =7

(4) η=1

(5) E (efficiency of the Huffman coding in reducing the bit rate perword close to the entropy of the data)=1.10

the average bit rate per word of the compressed data, regardless ofnumber of bits/word to be compressed, will be:

for data in square root format, b/W=3.98;

for data in logarithmic format, b/W=5.18; and

for data in linear format, b/W=5.96.

Processor Configuration and System Phase Operation

The system general block diagram, as shown in FIG. 1, includes aprocessor 124 providing a multitude of image reconstruction filteringand processing functions, now described. The operation of the processor124 is tailored according to the various processing functions. Theprocessor 124 is reconfigured according to a cross-point 150 switchmatrix including a plurality of switch devices forming a bi-directionaldata bus 145 between the system elements. The various elements of theprocessor 124, including the conversion look-up tables 128, 130, 132 and134, contained within the arithmetic unit 126, and the buffer memories138, 137 and 136 as well as data compressor 140, are engaged accordingto cross-point switch 155 within cross-point 150, shown in FIG. 2. Thebi-directional data flow through the plurality of cross-point switch 155comprise an arrangement providing the exchange of information among theplurality of units connected thereto, and is not necessarily limited toa particular or preferred form of hardware data connections orarchitectural distribution configurations.

The use of different formats of digitized data in different parts of thesystem provide cost savings without affecting the precision of theresults beyond any wanted value. Also, it is possible to easilytransform data through the use of look-up tables among differentformats, as shown in Table I, each requiring a different number of bits,without any relevant loss of information.

                  TABLE I                                                         ______________________________________                                        linear      to             √                                           logarithmic to             linear                                             logarithmic to             √                                           linear      to             logarithmic                                        √    to             linear                                             √    to             logarithmic                                        ______________________________________                                    

Relevant loss of information being defined as a maximum digitizationerror as compared to a given fraction of the intrinsic noise of theinformation regardless of how many times the unprocessed informationchanges its format. In fact, if a given number of different codesproperly describes the information encoded with Z bits, that same givennumber of different codes will sufficiently describe the sameinformation encoded with a higher number of bits. Specifically, when thelook-up table converts data from a format of a higher number of bits toa format of a lower number of bits, it will convert some different[adjacent] codes in an identical code. The information filtered out fromthis conversion truncation will not affect the precision of theinformation beyond the wanted portion of the noise but rather can berelated to the additional entropy of the signal noise. Conversely, whenthe look-up table converts data from a format requiring a lower numberof bits to a format requiring a higher number of bits, it will convertone word in a unique word of higher number of bits, and all of which arerequired to bound the digitizing error to a wanted fraction of noise ofthe signal and at the other end of the range of the signal, one word ina word of higher number of bits, with a certain number of leastsignificant bits arbitrarily set to a mid-point value between theadjoining values. It should be noted that setting those leastsignificant bits to the mid-point value therefor results in a reductionof the entropy of the noise and therefor, the contribution of noise tothe data. In a sense, the parameter η, previously defined, becomes aparameter of the entire x-ray system 100 and allow to optimally definethe word size of the memories and of the register of the arithmeticunits. Not shown in Table I, but within the scope of the presentinvention, are conversion look-up tables between the above-mentionedformats and other or general formats, also called x-formats.

The processor 124 operates in a plurality of phases, each defining aparticular hardware configuration implemented by closure of thecross-point switches in the cross-point and such phases may be groupedinto two basic categories: the first two phases are preprocessing phaseswherein the information received from digitizer 125 output is processedwhile it is being produced in real time, and thereafter stored in eithermemory or disc storage device; and the remaining phases being defined aspost-processing, wherein the information processes exists only in therecorded or stored memory devices. A typical sequence of phases fromboth groups is shown and discussed in connection with FIGS. 11-16inclusive.

Each memory 136, 137 and 138 comprises at least a matrix 512×512 points,or picture elements (pixel). Each pixel stores data having 8 or morebits, giving the memory three dimensions 512×512×8 bits. Each group of512×512×1 is called a bit-plane, thereby requiring at least 8 bit planesper memory. Each bit plane typically comprises four inch high densityRAM memory integrated circuits, having a capacity of 64×2 ¹⁰ bits. Thefour memory RAM used per bit-plane are read sequentially to form ahorizontal sequence of pixels as the video image is displayed on thevideo display 164, which scans from left to right. Also, the sequentialreading of each of the four RAM's per bit-plane into four differentregisters 412A, 412B, 412C and 412D of FIG. 11 allow the processor toperform the various phase operations in a pipeline sequence of four perarithmetic operations. The four pipeline registers 412A, 412B, 412C and412D, collectively called 412, are used throughout the phase operationsdescribed below to permit the sequential reading of memory data into thearithmetic unit 126, and, for the purpose of brevity, will be includedin the subsequent phase operatons without specific description.

Similarly, pipelines and delay registers 411 provide sequentialcollection of process data for each of the four pipeline sequences, topermit the appropriately connected memory 136, 137 or 138 to receive andstore the data. Additional registers 411A, 411B, 411C and 411D areincluded to permit the data to be shifted left or right by severalpipeline sequence steps, which occur at the rate of pixel calculationand display (≃10 MHz) to allow horizontal translation of the images(adjust picture registration) by selection of the appropriate number ofregisters to the left or right from which the memories receive data.

The first phase, as shown in FIG. 11, receives the digitized signal fromthe digitizer 125 at input port 146 into the processor 124. This leadcomprises 8 bits of data which is directed to the square root to loglook-up table within arithmetic unit 126 by engaging an appropriatecross-point switch 155 with cross-point 150. In addition, the image isdisplayed by display device 164 by engaging the appropriate cross-pointswitch, as above. The appropriate cross-point switch 155A, 155B, 155Cand 155D is determined according to the intersection point of the datasignal leads as shown in FIG. 2, and for the purpose of clarity, willnot be otherwise specified. The image is viewed in the square rootcompressed data mode. The look-up table within arithmetic unit 126provides a 12 bit output wherein it is stored initially by a registerwithin the arithmetic unit 126. Registers are included within eachelement of the arithmetic unit 126 to receive the incoming data signalsand store the resulting data signals as processed by each element of thearithmetic unit 126. The technique of storing or latching information inregisters before and after such arithmetic operations is known, and forthe purpose of clarity, the registers and their respective operationsare implied, and will not be specifically mentioned hereafter. The maskmemory 136 (or 137 or 138, if selected) stores all previous digitizedimage information which comprises the mask. When the first image isreceived by signal input port 146 of the processor 124, there is no maskimage stored in the memory 136. However, thereafter, there will be theprevious mask image stored therein. The previous mask is retrieved andreceived by the square root to log look-up table by engaging first thecross-point 150 and the pipeline registers 412. The look-up tableconverts the received square root signal to a 10 bit logarithmic signal.The converted logarithmic number from look-up tables 128 and 129 aremultiplied by I, where K is the number of images stored and (K-1/K) byadding the logarithmic equivalent of each, respectively in adders 408and 409. In the case of the first image received, K would be equal toone. Thereafter, the signal is converted to linear format by look-uptables 135 and 136, and added at summation block 410. The resulting sumis converted to square root format by table 132 and shifted sequentially(according to the above-described pipeline operation) into registers411A, 411B, 411C and 411D, to be read and stored by memory 136 as themost current value of the mask image M. When the mask image sequence iscomplete, that being a time period between 202 and 204 shown in FIG. 3,the averaging routine thus far described in accordance with Phase I isterminated and the mask memory 136 now contains an estimate of the maskvalue. This value is transferred to the disc 142 via the data compressor140 which reduces the number of bits from 8 bits of information providedby the mask memory 136 to a variable length code having an average valueof 3.9 bits per word, as discussed above in relation to Huffmanencoding. The memory is connected to the data compressor 140 byappropriately engaging cross-point 150 to complete the data transferpath. The disc now stores a compressed representation of the mask beingalso encoded according to a square root representation.

Phase II, as shown in FIG. 12, receives the images I_(J) as encoded bythe square root digitizer 125 at signal input port 146 of the processor124. The 8 bit signal received on the signal input port 146 isthereafter transferred through the cross-point 150 to a look-up table128 to produce a 10 logarithmic bit code to be received by thearithmetic unit 126. The mask memory 136, currently containing the valueof the average mask, is connected to a square root to log look-up table129 wherein the 8 bit square root signal is converted to a 10 bit logsignal. The arithmetic unit performs a subtraction at the summer 410 toproduce the difference between the mask image M and the images I_(J)occurring after the onset of the bolus, to provide the contrasted imagesΔ_(I) showing the blood flow and the arterial system or other images asdesired by the location of the unit on the subject. The resulting signalfrom the summer 410 is an 11 bit signal which is offset by an additionof a fixed value at 426 and multiplied by a predetermined value at 428to provide an improvement in contrast. The 8 most significant bits ofthe resulting 11 bit number is connected to the display 164 bycross-point 150. The images Δ_(I) displayed are in the logarithmicformat. Simultaneously, in Phase II, the 8 bit signal from buffer memory138 is received by the arithmetic unit 126 in an unconverted format byengaging cross-point 150 to transfer the information to pipelineregister 412. Similarly, the mask memory 136 produces the square rootformat of the old mask M_(o) and transfers this information to pipelineregister 413 (similar to pipeline register 412) of the arithmetic unit126 by engaging cross-point switches 158 and 157. The resulting signalsrepresenting the square root format of the old mask M_(o) and thecurrent image I_(J), are subtracted by summer 440. It should be notedthat, due to the fact that the incoming frame I_(J) encoded in squareroot format has only 8 meaningful bits, the 8 bit word size of thememory storing the difference between the incoming data and the maskM_(o), is adequate to prevent any truncation error resulting in aninformation degradation beyond the wanted level of precision. The resultis stored temporarily in pipeline and delay register 411. Thereafter, itis transferred through a memory 136, 137 and 138 to the data compressor140 and the disc 142 by engaging switches in the appropriate cross-point150. The disc thereafter will have a sequence of compressed mask imageM, and difference of images--the old mask, both encoded by the squareroot format and the variable length coding incorporating both spatialand temporal encoding and therefor having an average value of 3.9bit/word as discussed above in relation to the Huffman coding. Theimages thus stored on the disc typically are a subset of those generatedby the x-ray system 100.

When the bolus has passed (206 in FIG. 3), the sequence of new images iscomplete and the preprocessing phases (I and II according to FIGS. 11and 12) are complete. The post-processing mode is now begun.

The first post-processing phase, Phase III, allows the replay of theframe of images I_(J) previously stored on the disc, as differenceimages Δ_(I) (in log format) against the old mask M_(o) (also in logformat). The replay is performed, as shown in FIG. 17A, in such a waythat the display shows the sequence of the differences Δ_(I) at a speedcontrollable by the operator, thus allowing a more careful analysis ofthe events as occurring during the process. More specifically, the frameor the differences can be frozen on the screen, under the control of theoperator, as well as replayed backward. During this part of Phase III inFIG. 17B, the images, stored during the processing in the disc incompressed format as differences I_(J) -M_(o) against the old mask M_(o)are automatically added by adder 408 to the same mask M_(o), thereafter,in FIG. 17C the images are sent to the date compressor 142 to berestored in the disc 140; as images I_(J), in square root format, nolonger dependent upon the old mask M_(o). The images thusly stored inthe disc 140 are only spacially compressed, no longer using the temporalcompression technique. At the end of this Phase IIIc, FIG. 17C, theimages stored in the disc, no longer dependent upon the old mask M_(o),can be used to display any combination of them, depending upon thevarious software routines that can be stored in the system controller.

Examples of post processing combination of images are the following:

First, the choice of an alternate mask is made. After the operator hasidentified a difference image, which presents relevant diagnosticinformation as far as the presence of the bolus is concerned, but alsopresents some artifacts due to the modification of the background (forinstance, due to some movement of the patient between the time intervalacross which the old mask and the intersecting image have been taken),the operator can fetch any other image stored in the disc, to be used asa new mask against which the interesting image is subtracted.

Second, the averaging of a new mask M_(N) is performed. The operator mayhave identified a certain number of images I_(K), all acceptable to beused as alternative masks. The operator can, at this moment, average theimages I_(K) to form a new mask M_(N), which being an average of acertain number of frames I_(K) will have a reduced noise, but also willshow, when substracted to the interesting image, minimized artifacts dueto the patient movement.

Third, time interval difference (T.I.D.) is performed. The operator canreplay the sequence of images so that the screen shows the differencebetween the log of a frame image I_(J) and the log of the frame imageI_(J-K), which is the image that has been stored on the disc during theprocessing, K steps before the image I_(J). The values of K can be setby the operator. The sequence of these new images is known as T.I.D.post-process.

Fourth, automatically detecting the optimum difference image. Based onthe calculation of average values of β_(J) per each frame, the systemcan be programmed to build two averages of the images I_(J) storedduring the presence of the bolus. The first average is built by weighingeach image I_(J) with a factor proportional to the average value ofβ_(J), and can be used as an optimal image. The second average is builtby weighing each image I_(J) with a factor inversely proportional to theaverage value of β_(J), and can be used as an optimal mask. In addition,after the Phase III is completed, the system can be programmed to modifyan image I_(J) fetched from the disc and stored in a memory into amodified image I_(J) * which is stored either in a different memory ofthe system or restored in the same memory. Examples of thesemodification are the following:

First, performing the reregistration of an image. The modified imageI_(J) * consists the displacement of the image I_(J) of any number ofpixels along the x- and/or the y-axis of the display. The modified imageI_(J) *, can be used to eliminate or minimize the image artifactsarising from patient movement which have taken place between thedetection of the mask and the detection of the I_(J).

Second, digitally filtering an image. A high-pass (to enhance the edge)or a low-pass (to smooth out the image and reduce the noise) digitalfiltering of an image I_(J), can be built and stored in a differentmemory of the system, by a sequence of translations, multiplication ofthe translated value by a factor (known as coefficient of the Kernell)and addition of the translated and multiplied value into another memoryof the system, that, at the end, will store the filtered image.

A specific example of the first post-processing phase, Phase III isshown in FIG. 13, which allows the frame F of image I_(K) to be selectedfrom among the stored images, either automatically from the onset of thebolus (point 204 in FIG. 3) backwards, or by choice of the attendingphysician. The selected image I_(f) is stored in memory 136 throughcross-point 150; the image, when stored, is subsequently transferred tothe processor 124. The interactive operation with the physician isprovided by coincidentially plotting over time, on the display 164, theaverage absorption (μ) and the occurrence of low probability signalswhich are not compressed by a Huffman code in the data compressor 140(as related according to the earlier discussion of FIG. 7) whichtherefor indicates a high degree of movement of the subject. This firstsignal is represented by the onset of the bolus as shown in FIG. 2. Thephysician or operator thereby observes a level of movement according toa proportional level of low probability signals (which are indicated bythe number of service words recorded) during the moments preceding theonset of the bolus (point 204 in FIG. 3) and chooses the signal moststable having the longest period of low activity over which to average.The disc 142, currently having the compressed old mask value M_(o), andthe image difference signal Δ_(I), is decoded according to the datacompressor 140 and received by the arithmetic unit 126 by engaging theappropriate switch 155 of cross-point 150. The data stored comprises adata word encoded according to the square root format. The buffer memory138, having the mask image M stored thereon, is received through thecross-point 150. The resulting images are added by the summer 408 whoseresult is stored in memory 136.

The average value of the mask according to the current image is computedaccording to Phase IV shown in FIG. 14. The images stored in disc 142are sequentially decoded by data compressor 140 and coupled to apipeline register 413 of the arithmatic unit 126 and in turn, to asquare root to log look-up table 128 of arithmetic unit 126. The look-uptable 128 converts the 8 bit data word to a 10 bit data word. Theprevious value of the mask M is stored in the mask memory 136 andconnected to a pipeline register 412, and in turn to a square root tolog look-up table 129. The resulting 10 bit data words are added withthe logarithmic equivalent of 1/K, at 402, and (K-1/K) at 403 by adders408 and 409, respectively. The results are converted to linear signalsby look-up tables 135 and 136, and added by adder 410. The results areconverted to logarithmic values by look-up table 132, which value is inturn received by pipeline and delay register 411. The register 411 isconnected to the memory 136 through cross-point 150. This current valueof M is then stored in place of the previous mask value and theabove-described cycle of averaging continues until the last image isprocessed in this manner. At that time, the mask memory 136 stores itsvalue for the current averaged mask in the disc 142 as encoded by datacompressor 140.

More generalized post-processing may be performed according to thesequence of Phase V shown in FIG. 15. A particular image F_(J), storedon the disc 142, is decoded by decoder 140 and connected to mask memory136 and is stored therein. The image F_(J) in square root format in themask memory 136, being either the image or the difference between theimage and the mask, is converted to log format, and then to the desiredformat by the look-up table which may include any of the above-mentionedlook-up tables by engaging the appropriate cross-point switch 155. Theresulting multi-bit word is received by the arithmetic unit 126 toperform the particular generalized process P in the x-format as desiredby the operator. The resulting signal is thereafter either viewed bydisplay 164 through cross-point 150 or converted from the x-format againto the log and then to the square root format by appropriate convertinglook-up tables. The signal is thereafter coupled to the buffer memory138 by engaging the corresponding cross-point switch 155. The signalstored by the buffer memory 138 represents the process function of theimage F_(J). The image therein stored may be displayed on the 512 by 512display device 164. Alternately, the unprocessed signal and theprocessed signal, which in log format, are subtracted by adder 410 andconnected to display 146 for system operator real-time observation ofthe processing P. The function or process performed on the image F_(J)may be also stored on the disc 142 when compressed by the datacompressor 140 by engaging the cross-point switch 154.

It is of note that the arithmetic unit 126 may be reconfigured accordingto the various phase drawings of FIGS. 11-15, 17A-17C as well asadditional configurations according to known processor designtechniques. These include additional registers, arithmetic functionssuch as multipliers, dividers and shift registers and other devices, aswell as other data paths within the arithmetic unit 126 according to thedesign parameters of the particular digital x-ray processor constructed.

It is also within the scope of the present invention to include othermodifications, additions and alterations as may be performed by oneskilled in the art and are hereby incorporated into the presentinvention, which is not to be limited accept by the following claims.

What is claimed is:
 1. A radiology system comprising:an x-ray source fordirecting radiation through an x-ray absorptive body; means forreceiving radiation from the source after passage through the body toprovide an analog signal in response to and representative of theintensity of the received radiation; analog to digital converter meansproviding a digital signal according to a digitization interval relatedto the intrinsic uncertainty of the signal, assuring that informationlost from digitization is less than a predetermined amount of signalinformation relative to the intrinsic uncertainty of the signal of theincoming analog signal; and means for storing the digital signal.
 2. Thesystem of claim 1 wherein said encoding means includes means forproviding temporal and spatial averaging of the electrical signal. 3.The system of claim 1 wherein said encoding means includes:means forproviding variable length coding of the electrical signal; and means forproviding temporal and spatial averaging of the electrical signal. 4.The system of claim 1 wherein said digitization interval is non-uniformover the range of said analog signal.
 5. The system of claim 1 whereinsaid means for receiving radiation includes means to provide a visibleimage.
 6. The system of claim 1 wherein the intrinsic uncertainty of thesignal includes signal noise having a noise distribution.
 7. The systemof claim 6 wherein the intrinsic uncertainty of the signal is related tothe square root of the analog signal, said analog-to-digital converteroperative to provide a digital output representing a square rootfunction of the analog signal.
 8. The system of claim 1 furtherincluding encoding means for providing variable length encoding of thedigital signal in real-time.
 9. The system of claim 8 furtherincluding:means for storing the variable length encoded digital signal.10. The system of claim 8 wherein said encoding means includes Huffmanencoding means.
 11. The system of claim 8 wherein said encoding meansincludes means for providing truncated Huffman coding of the electricalsignal.
 12. The system of claim 11 including means for decoding thedigital signal.
 13. The system of claim 12 including means fordisplaying the decoded digital signal.
 14. In an x-ray radiology systemin which an x-ray absorptive dye is injected into a body being subjectedto x-ray radiation, the improvement comprising:an x-ray source fordirecting controllable levels of radiation through an x-ray absorptivebody; means for receiving radiation from the source after passagethrough the body to provide an electrical signal in response to andrepresentative of the intensity of the received radiation; means forproviding a control signal upon predetermined change in the electricalsignal;and means for automatically controlling the x-ray source inaccordance with the control signal to provide a lower level of radiationuntil the average signal changes by the predetermined amount, and ahigher level of radiation after such change.
 15. The system of claim 14wherein said means for receiving radiation includes image conversionmeans in which the gain is adjusted according to the particular level ofx-ray radiation received, the gain adjustment being inversely related tothe x-ray radiation intensity.
 16. The system of claim 14 furtherincluding:means operative in response to the electrical signal forproviding an average signal representing the average intensity of thereceived radiation, said means for controlling the x-ray source beingresponsive to said average signal.
 17. The system of claim 14 furtherincluding:means to form an image I from the received radiation.
 18. Thesystem of claim 17 further including:means to derive a mask image Mafter said predetermined change in the average signal.
 19. The system ofclaim 18 wherein said means to derive a new mask M_(N) from an averageof a plurality of images I.
 20. The system of claim 18 furtherincluding:means to form a difference in image from the mask image M andthe image I.
 21. For use with a time sequence of images having anintensity range, signal conditioning means comprising:means forproviding an electrical signal representing temporal and spatialvariations in the intensity of the images; analog-to-digital convertermeans for providing a digital signal representing the square root of themagnitude of the electrical signal; means for providing temporal andspatial data compression of the digital signal and an encoded digitaloutput signal representative thereof; and means for storing the encodeddigital output signal.
 22. The signal conditioning means of claim 21further comprising means for decoding the stored encoded digital outputsignal.